Lithography has been utilized for semiconductor device fabrication for decades. Radiation-sensitive resist material is deposited on a semiconductor workpiece and it is exposed to radiation from an energy source through a patterned mask. The workpiece is subsequently processed (including development of the resist, selective removal of either the developed resist or of the non-exposed resist material, deposition of metal in selectively-exposed areas, etc.) to create a pattern of features needed for the semiconductor device. The implementation of lithography in the semiconductor field has advanced from the use of photolithography for larger-scale features to the use of shorter wavelength radiation sources to fabricate higher density circuitry having smaller features. Synchrotron X-ray radiation is one of the preferred energy sources for providing high density patterns of features on semiconductors. Due to the fact that synchrotron X-ray radiation is produced as a broad horizontal beam having a small vertical spread, the wide beam is scanned along the length of a mask overlaying a workpiece to expose successive vertically-disposed sites on the workpiece. The use of the term "vertical" is used herein in accordance with the common parlance in the industry. It will be clear to those having skill in the art that use of the term "vertical" does not mean that the workpiece could not be mounted at any orientation other than vertical in the exposure system; but, rather that the scan is conducted perpendicular to the plane of the mask to effect irradiation through all of the mask feature areas.
In X-ray lithography systems, of the type illustrated in FIG. 1, radiation from the synchrotron source, 13, is first directed to a mirror, 15, at a shallow angle of incidence, .alpha.. The incident angle, .alpha. is chosen to be in the range of 25 mradians in order to maximize reflectivity of the radiation. The reflected radiation is provided at the patterned mask, 11, and, through the X-ray transparent sections of the mask, to the resist-coated workpiece, 10. In order to scan the radiation vertically along the mask and the workpiece, hereinafter referred to as the "mask-workpiece combination," the mirror may either be rocked, so as to change the angle of incidence of the radiation, or it may be displaced vertically while keeping the angle of incidence constant. Rocking of the mirror is an option, however, since the reflectivity of the mirror is a strong function of the angle of incidence, the intensity of the reflected beam can vary as the beam is scanned vertically over the mask-workpiece combination. Such variation in the intensity of the beam across a workpiece may lead to unacceptable variations in the amount of exposure. Using vertical displacement of the mirror, such that the X-ray radiation is provided at various points along mirror length L, and the reflected radiation is vertically displaced along a section of the face of the mask-workpiece combination, provides a predictable amount of radiation across the entire surface of the mask-workpiece combination.
FIGS. 2a through 2c show, in schematic form, the scanning operation of the prior art systems. When mirror 15 is in a first position, P.sub.a, with respect to the synchrotron X-ray source, the incident beam is reflecting off of the mirror at a point M.sub.a, and being provided to the mask-workpiece combination at a point W.sub.a, as illustrated in FIG. 2a. As the mirror is "vertically" displaced, in a direction perpendicular to its planar mirror face, to the position denoted at P.sub.b in FIG. 2b, the incident beam is contacting the mirror at point M.sub.b and being provided to the mask-workpiece combination at point W.sub.b. Finally, when the mirror is located at position P.sub.c, as in FIG. 2c, the X-ray beam contacts the mirror at point M.sub.c and the mask-workpiece combination at W.sub.c. While there are many positions along the mirror length, L, positions P.sub.a, P.sub.b and P.sub.c are representatively depicted to illustrate the fact that while the radiation has been caused to traverse along mirror length L, the reflected beam has covered a continuously illuminated area of vertical height V on the mask-workpiece combination, which area may incorporate one or several mask features.
Because the vertical height scanned by the beam is given by the relation V=L.times..alpha., also stated as V/.alpha.=L, and the angle of incidence .alpha. is very shallow, the length L of the mirror must be approximately 2000 mm in order to scan the reflected light across a workpiece dimension V of 50 mm. For accurate reflection of the radiation, the mirror must be perfectly planar over its entire length, which presents significant technical challenges, particularly at the aforementioned dimensions.
Prior art patents U.S. Pat. Nos. 5,234,626 of Flamholz, et al and 4,260,670 of Burns, both teach the use of masks having multiple apertures or mask pattern segments separated by opaque regions for exposing multiple chip sites. The Flamholz, et al patent additionally teaches use of the segmented mask in an X-ray scanning system. An improved mask structure has been disclosed in U.S. patent application Ser. No. 08/731,536 entitled "MEMBRANE MASK STRUCTURE, FABRICATION AND USE" by R. Acosta and R. Viswanathan, wherein one embodiment of the inventive mask comprises a two stage mask for two-step exposure, with the chip site patterns at each of the stages being separated by support segments. One drawback to the use of any of the foregoing masks is the fact that time and energy are being wasted while the radiation is scanning over opaque areas such as the support sections of the masks.
FIG. 3 shows a mask, 30, having four aperture segments, 31, 32, 33 and 34, separated by opaque regions. For ease of description (as well as for ease of fabrication and use in practice, though such is not strictly required), each of the mask segments in equal in vertical dimension v, and in separated by an opaque area of equal vertical dimension s. The indicators on the left of FIG. 3a represent the time intervals during which the scanned, reflected X-ray radiation is traversing each mask segment and each opaque region. Time intervals T.sub.1, T.sub.3, T.sub.5 and T.sub.7 each represent the time taken to traverse the mask pattern aperture segments 31, 32, 33 and 34, during which times the X-ray radiation is reaching the workpiece. During all of the other indicated T.sub.x intervals, the energy is being wasted, and the ultimate throughput of the system is being diminished. The vertical dimension of the entire mask feature area, including the opaque segments, is denoted as V, indicating the area traversed by reflected radiation from a scan of a mirror of length L.
The time periods are shown as equal, however, it is to be noted here that it is not required that each of the time periods represented by T.sub.x be equal in duration, or that the mirror scan be conducted at a constant rate. One solution to the time and energy waste would be to program a system to increase the rate of progress of the reflected beam over the opaque regions of dimensions s in order to minimize the compromised throughput. Such programming would be burdensome to enter into the system for each mask, and would be quite difficult to control.
In light of the foregoing discussion, it is desirable to provide a reflection system for X-ray lithography tools which would not require the use of large mirrors, which would optimize throughput time, and which would minimize waste of energy.
It is therefore an objective of the invention to provide an X-ray lithography mirror system having segments of relatively small dimensions in comparison to prior art, non-segmented mirrors.
It is another objective of the invention to provide an X-ray mirror lithography system which directs the radiation to the patterned feature areas of the mask, thereby avoiding unnecessary waste of energy over non-patterned opaque mask regions.